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In this state-of-the-art review, various adsorbents (i.e., Metal-Organic Framework (MOF), Activated Carbons and their composites, Carbon Molecular Sieve (CMS) and zeolites) for CO2 adsorption both at low and high-pressure applications (i.e., pre- and post-combustion CO2 capture, adsorption heat pumps employing CO2 as refrigerant) are explored. The most suitable candidate, i.e., the various grades of activated carbons (ACs), is identified based on their equilibrium uptake (isotherm data), rate of adsorption (kinetic data), isosteric heat of adsorption and cost. The study presents a comprehensive review on the basis of various models of adsorption isotherms and kinetics, their merits and demerits, and their applicability, especially in the context of CO2-adsorbent pairs. The literature shows that the activated carbons with high surface area, pore volume and better pore network results in higher equilibrium uptake and faster kinetics. A comparative analysis presented in the review work highlights that high-grade activated carbons having higher absolute uptake, also result in higher net uptake, i.e., the deciding factor for selecting the adsorbents for adsorption-based refrigeration and heat transformation applications. The comparative study clearly shows that most of the MOFs with high surface area outperform the best-activated carbons in equilibrium CO2 uptake. However, their high heat of adsorption, slower kinetics and significantly high cost comes in their way of commercialization for high/low-pressure CO2 adsorption applications. One of the notable observations of the review work is that adsorbents that perform better in low-pressure applications may not be a handsome candidate for high-pressure applications, as both mechanisms are different. Various isotherm models are compared based on the R2 value of the fitted data. The comparison clearly demonstrates that some of the models, i.e., (Langmuir, Freundlich), give better predictions at low-pressure conditions while some (Toth, D-A, Modified D-A, and D-R) give a better prediction for high-pressure adsorption. Some isotherm models take care of the surface heterogeneity, hence suitable for AC-CO2 pair. Most importantly, the compiled data for the adsorption isotherms and kinetics will be useful for further analysis and design of adsorption systems and selection of adsorbents, especially for CO2 adsorption systems suitable for green refrigeration/heat pump and carbon capture application. [ABSTRACT FROM AUTHOR]

• An in-house adsorption test rig is designed and developed for CH 4 -AC pair. • Isotherm and kinetic constants are determined for subzero and high temperatures. • Physiochemical and thermophysical properties are experimentally estimated. • Heat of adsorption, adsorbed phase-specific heat, enthalpy, entropy are estimated. • 3D transient analysis of the ANG reactor is conducted. The current study investigates the equilibrium methane (CH 4) adsorption capacity and the adsorption kinetics of a locally produced coconut shell-derived activated carbon (AC CARB 6X12 60) for pressure and temperatures ranging from 0 to 55 bar and -20 to 70 °C, respectively. The adsorption isotherms and kinetics are determined using an in-house developed adsorption test rig. A maximum methane adsorption uptake of 0.140 kg/kg is obtained at -20 °C and 40 bar pressure. The adsorption equilibrium isotherm data are fitted using Toth, Dubinin-Astakhov (DA), and Modified-DA isotherm models. Afterwards, the kinetics study is done considering pseudo first order, second order, Elovich, and intra-particle diffusion models. In continuation, based on the DA isotherm model, Clausius Clapeyron relation, and Maxwell's relations, the expressions for the crucial thermodynamic parameter of adsorbed phase, i.e., isosteric heat of adsorption, specific heat capacity, enthalpy, and entropy is derived, and same is estimated using DA isotherm parameters. Additionally, the activated carbon's thermophysical properties (i.e., thermal conductivity, thermal diffusivity, and specific heat) are determined using the TPS 2500 S analyzer, which works on the hot-disk principle. Using the most suitable isotherm (DA) and kinetic model (pseudo second order), heat and mass transfer analysis is carried out on novel reactor configuration with an external cooling jacket, double row of cooling pipes with external longitudinal fins for methane storage using a 3D transient model in Comsol Multiphysics. The charge characteristics are studied for gas charging pressure varying from 1 to 50 bar and cooling fluid temperature of -20 to 70 °C. Iso-concentration contours are plotted to identify the pressure-temperature combination resulting in identical storage capacity (i.e., including both gaseous and adsorbed phases). Further simulations are carried out for constant pressure charging and constant flow discharge conditions to expound upon the effects of the dormancy period, gas charging pressure, and cooling/heating fluid temperature. 14% improvement in total discharge amount is obtained with a dormancy period between charging and discharging compared to no dormancy case. [ABSTRACT FROM AUTHOR]

A novel micro-mesoporous activated carbon (SBL AC-700) is synthesized from subabul (Leucaena leucocephala) sawdust waste by direct single-stage physical activation at 700 °C for 1 h for carbon capture applications. The synthesized AC is characterized to explore various physiochemical properties like elemental composition, surface morphology and crystallinity, presence of functional groups, surface area, pore size, and pore volume. Additionally, emphasis is given to exploring the thermophysical aspects of the novel AC, the literature regarding which is scarce in the open domain. The CO 2 adsorption study is carried out for a 0–1 bar pressure for temperatures ranging from 0 to 75 °C. The analysis revealed that the AC possesses a surface area of 590 m2/g and pore volume and width of 0.27 cm3/g and 1.85 nm, corresponding to a 70% microporosity with a well-developed porous structure. At 25 °C and 1 bar, a CO 2 uptake of 40.54 cm3/g is achieved, corresponding to an increment of 6–202% compared to other commercials, chemically and physically activated carbons. Moreover, SBL AC-700 has a thermal conductivity of 0.095 W/m K, 8–131% higher than other benchmarks ACs and much lower specific heat of 0.82 kJ/kg K corresponding to lesser regeneration energy requirements. Experimental data are fitted with various isotherm models, i.e., Langmuir, Freundlich, Sips, R–P, and Toth, out of which R–P and Toth models exhibit the best fit. In continuation, adsorption kinetics is studied to explore the dynamic performance of the SBL AC-700 by using Pseudo-first-order, Pseudo-second-order, Elovich, and intraparticle diffusion kinetic models. [Display omitted] • AC SBL AC-700 from subabul sawdust waste is synthesized successfully. • Novel AC is prepared by eco-friendly single-step direct CO 2 activation. • The ability of CO 2 uptake at 25 °C and 1 bar is 40.54 cm3/g. • SBL AC-700 has a thermal conductivity of 0.095 W/m K, 8–131% higher than other ACs. • Isotherm, kinetics, and thermodynamic modeling is done for the AC-CO 2 pair. [ABSTRACT FROM AUTHOR]